Cleaning showerhead holes is a pain.  No need to use harsh poisonous chemicals. An easy way to do it is by unplug the shower head and soak in a mix of equal parts of white vinegar and warm water for about 2 hours. Rinse it off.

A mix of 2 tablespoons of white vinegar with 1 quart of warm water works well on bathroom tile.  Be sure to use a soft towel and rinse it off afterwards.